Re: [Pgcluster-general] PostgreSQL Documentation of High Availability

Hi,

Bruce Momjian wrote:
> I feel the shared-* issue splits us up like master/slave and
> multi-master splits up

No, not quite. To sum up, I'd say the following combinations make sense:

sync, multi-master replication on shared-memory cluster (which is much
like a super-computer. With shared memory distributing locks does not
cost much - beside marketing, there is probably not much sense in
calling this a cluster at all).

sync, multi-master replication on shared-disk cluster (where locks and
memory-caches have to be synchronized. OracleRAC and PgCluster-II fit in
here.)

(Probably running an async replication on a shared-disk cluster would
make sense with MVCC and in some corner cases, but I don't see much
benefits in that.)

sync, multi-master replication on shared-nothing cluster (where locks,
caches and data needs to be synchronized over an interconnect.
Postgres-R, PgCluster, PgPool)

(sync, single-master replication does not make much sense, because if
you go sync at all, you could as well use the nodes which run in sync).

async, multi-master replication on shared-nothing cluster (i.e. Slony-I)

async, single-master replication on shared-nothing cluster (mainly for
failover purpose, you mention solutions for that)


For me these categorizations are important and help a good deal to
ensure what I'm talking about with somebody. The documentation is much
more focused on individual solutions, sometimes avoiding to categorize
them. I would love to get others opinions, but as not many others speak
up, I just accept it that way.

> Yea, gets confusing.

Well, Oracle also does a good deal in making it confusing, IMO.

> Good point. I mentioned Oracle RAC only because it seems to be an
> industry standard, so by mentioning it, people know exactly what we are
> talking about.

That's a point, even if I don't really know how much of an industry
standard it is. But given how badly Oracle does in explaining basics of
replication and clustering, I think it's not very beneficial.

> Is there a better way?  And people do ask for Oracle
> RAC, so in a way we are telling them we don't have something similar.
> As sad as that is, it is true currently.

How far is PGCluster-II? Does it make sense to mention it? Can
PGCluster-II be used with network filesystems like NFS, OCFS2 or the like?

> pgcluster is must closer to Oracle RAC,

Why do you think so? Oracle RAC is mainly based on a shared disk
cluster, where PGCluster bases on a shared nothing architecture.
PGCluster-II seems closer to Oracle RAC, for me.

> but I haven't mentioned it
> because I am unsure where it is in terms of usability and stability.
> Comments?

Did you work on it since Toronto, Mitani-San?

Regards

Markus